Every industry that deals with heat transfer strives to simplify and reduce the size of the apparatus employed to perform the heat transfer function, while improving heat transfer efficiency. For instance, the commercial baking industry desires to improve the convective heat transfer while cooling a baked product on a moving belt. Often the belt with the baked product is placed inside an enclosed channel, generally referred to as a cooling tunnel. The cooling tunnel usually has a rectangular cross-section and is very long in nature. The product to be cooled usually travels along the bottom of the tunnel. Air or other gases for cooling are forced along the top of the cooling tunnel by a fan to effect the heat transfer. Due to the long nature of the cooling tunnel, it is always desired to find ways to reduce the cooling tunnel length. Other industries use such channels to convey heat as well as remove it. Typically such heat transfer channels of this nature are referred to as cooling tunnels, heating tunnels, cooling channels, ovens and so on. In this discussion and the claims included hereinafter, these types of channels will be collectively referred to as heat transfer tunnels. The gaseous medium used to effect the heat transfer can be any gas desired for the purpose of heat transfer. In most cases the gas used for heat transfer will be air and therefore all gases that can be use will be collectively referred to as air in this discussion and the claims included hereinafter.
Convective heat transfer is governed by Newton""s Law of Convection, which can be written as q=Q/A=h(Tsxe2x88x92T∞). Where q is the heat flux (rate of heat transfer per unit area); Q is the rate of heat transfer; A is the surface area to or from which heat is being transferred; h is the convective heat transfer coefficient; Ts is the surface area temperature; and T∞is the ambient air temperature far from the surface area, usually towards the top of the heat transfer tunnel. From the above equation, the only way to increase the heat flux from the surface area to be affected by the heat transfer is to either increase h or increase the temperature difference (Tsxe2x88x92T∞).
A current method of enhancing heat transfer in a heat transfer tunnel is the method of impingement heat transfer. Impingement heat transfer is the directing of air through many air jets which are aimed directly onto the surface area of the product to be heated or cooled. The convective heat transfer coefficient depends strongly on the lateral distance from the impinging air jet as shown by the graphs in FIGS. 1 and 2. FIG. 1 shows the distribution of the convective heat transfer coefficient as a function of distance from jet centerline for a large nozzle-to-surface area spacing. FIG. 2 is the same as FIG. 1, but for a small nozzle-to-surface area spacing. Accordingly, a large number of relatively closely spaced jets are required to heat or cool a commercial product. This method is expensive due to the large number of impinging jets that are needed to provide heating or cooling in commercially sized heat transfer tunnels. It is difficult to provide an effective distribution of the air flow to the nozzles for these jets. Also, there is the requirement to remove the xe2x80x9cwastexe2x80x9d air after it impinges vertically on the surface area of the product without disrupting the desired impinging jet flow pattern.
It is an object of the present invention to provide an apparatus and method to redirect airflow and enhance heat transfer using an oscillating baffle. It is also an object of the present invention to provide an apparatus and method to improve the efficiency of a heat transfer tunnel, while reducing the size of the tunnel.
The present invention is an oscillating baffle and method to redirect airflow and enhance heat transfer. The oscillating baffle is a baffle having a length between two ends, and a height and width which form a cross-section of the baffle. The baffle has a first degree of freedom and a second degree of freedom which allows it to oscillate under the power of the airflow. There are several embodiments providing the first and second degrees of freedom that are disclosed. The method employs an oscillating baffle to redirect and mix the airflow in order to enhance heat transfer in a heat transfer tunnel.